Certain embodiments of the present invention relate to the field of radiation therapy and radiation dose measurement for all types of external ionizing radiation, including photons, proton beams, heavy ion beams, and for internally-administered radioactive isotopes and x-ray sources.
The properties of plastic and organic scintillators are known to be ideal for radiation dosimeters used in radiation therapy. A series of seminal papers published in the early 90s have established that plastic scintillation detectors have a unique set of advantages such as water-equivalence, dose linearity, dose rate independence and energy independence under irradiation by megavoltage photon and electron beams. These scintillators can therefore be used for precise and accurate measurements of radiation doses involved in radiation therapy treatments. The ideal characteristics of plastic scintillators first prompted their use as small, single-point dose detectors, and then as arrays in large detector systems. Two-dimensional systems have also been proposed by using continuous sheets of scintillating material.
Fast and accurate measurement of radiation doses in multiple points or in a plane is useful, but as the capacity for conformal radiation beam delivery increases, one is confronted with complex dose patterns following the shape of a tumor in three dimensions. Furthermore, such complex dose patterns are often delivered dynamically either with rotational treatment modalities such as volumetric-modulated arc therapy (VMAT) or intensity modulated proton therapy (IMPT) delivered with scanned pencil beams. Such treatment modalities are among the fastest growing approaches for delivering highly conformal doses of radiation with curative intent.
A fast, three-dimensional dose detector would allow a complete mapping of the radiation dose no matter how complex. At the present time, only dosimetric gels can be used to make thorough 3D dose measurements. Dosimetric gels are either based on the behavior of ferrous ions or on the polymerization of a monomer in response to irradiation. A large variety of chemical formulas are available for the gels, and each has its own set of advantages and disadvantages. However, they usually share a delicate fabrication process; they require time-consuming post-processing manipulation and analysis; and they may suffer from various artifacts. These characteristics make the daily clinical usage of dosimetric gels somewhat challenging. In addition, dosimetric gels measure only the integral dose from an entire treatment delivery. They are not capable of measuring the time-dependent aspects of dose delivery.
Aside from dosimetric gels, arrays of radiation dose detectors (ionization chambers or diodes) have been developed to partially map complex dose distributions. However, for an accurate measurement of a three-dimensional dose distribution, the detector itself must not alter the passage of ionizing radiation. For radiation therapy applications, we are interested in the interaction between ionizing radiation and materials similar to water and/or human tissues. The use of detector arrays is therefore not ideal because such detectors are either based on silicon diodes or air-filled ionization chambers, both of which significantly differ from water and human tissues. Consequently detector arrays cause perturbations of the radiation fields. For this reason, most detector arrays are solely used to monitor radiation in the plane perpendicular to the radiation beam. Furthermore, detector arrays often suffer from intrinsic limitations. The size of the detector elements might limit the spatial resolution of the measurements (e.g. the ionization chambers used in arrays often have a detector size of more than 5 mm). The minimum possible spacing between detectors might also create gaps where no information can be obtained.
Because of the inherent water equivalence of plastic scintillators, a detector made of this material is similar to water and human tissue and does not perturb an incident radiation beam as it travels through the device. A scintillator-based prototype for measuring 3D doses produced by eye-plaque brachytherapy applicators has been reported. This prototype performs a tomographic reconstruction of the dose by rotating a vial of liquid scintillator containing the eye plaque. This prototype was designed only for measuring dose distributions of small volumes (<20 cm3) and required long acquisition times (more than 5 hours for 64 projection angles). However, this work proves that scintillator based system can be used for 3D dose measurements.
Recently, a volumetric dose detector based on liquid organic scintillator was developed. This liquid scintillator (LS) detector device is imaged by a charge-coupled device (CCD) camera. A radiation beam incident on the LS produces a pattern of scintillation light. This pattern is a function of the amount of radiation dose deposited. The CCD camera placed at one side of the cubic LS volume captures images of this light distribution. The LS detector system is not a 3-D dosimeter per se because the CCD images represent the integral of all the light produced in the volume along the camera axis. Even if the images contain information about the entire 3-D dose distribution, it is impossible to extract the dose delivered at a given point. The LS detector system was developed for two main applications: one for proton therapy and one for intensity modulated radiation therapy (IMRT). For proton therapy the LS detector system was irradiated with passively scattered beams and magnetically scanned proton pencil beams. We have shown that with scanned proton beams, the lateral position and the depth (which is directly correlated to the energy of the protons) of individual beams can be measured with sub-millimeter accuracy. For IMRT the LS detector system was used as a verification device. Clinical treatment plans were transferred from the patient CT to the CT dataset of the LS detector. Then a forward projection of the planned dose was made to simulate the expected scintillation light distribution. Finally, the simulated and measured light distributions were compared for every segment of every beam to detect any discrepancies that could have occurred during the treatment.